After a stretch of relatively dry and mild conditions, a strong low pressure system impacted the Great Lakes bringing the return of winter conditions. Near-record high temperatures were met with rare December thunderstorms as the low pressure system rapidly strengthened as it approached the area. As the storm passed by, high winds with gusts over 60 MPH lead to sporadic power outages and heavy lake effect snow across the west and northwestern snow belts. A hurricane-force wind gust of 75 MPH was also measured at the Stannard Rock Lighthouse on Lake Superior.

Gusty winds were observed across the entire Upper Peninsula on Tuesday, December 5th. The strongest winds, at times gusting over 60 MPH, were confined to Lake Superior and the western shore of Lake Superior/along the Keweenaw Peninsula. The strongest winds were reported at Stannard Rock Lighthouse, with sustained winds of 60 MPH and gusts in excess of 70 MPH. More information about winds over Lake Superior are included in the "Marine Weather" tab.

Here is a list of the highest wind reports received during the day on December 12th.

The highest winds for this event were measured on Lake Superior at the Stannard Rock Lighthouse. The station reported wind gusts in excess of 60 MPH from ~4:30 AM 12/5 to 3:00 AM 12/6--over 22 hours straight! At times, the wind was sustained at 60 mph with gusts in excess of 70 MPH! The peak gust at Stannard Rock Lighthouse was 75 MPH at ~ 7:00 PM 12/5.

Below are plots of the wind speed (knots, blue), wind gusts (knots, red), and air pressure (inches of mercury, green) courtesy of the National Data Buoy Center.

Stannard Rock

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Caribou Island (Canada)

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Passage Island - Isle Royale

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Rock of Ages - Isle Royale

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Snowfall Reports:

While some snow fell along the backside of the system, the majority of the snow from this event will fall as lake effect. As of now, here are the highest wind reports we've received.

The maps below from the surface through 300 mb (~ 30,000 ft) demonstrate the evolution of the strong and dynamic storm that tracked across western Lake Superior late Monday, Dec. 4th through Tuesday Dec. 5th.

As can be seen in the following three maps at 300 mb (~ 30,000 ft), two upper-level jet streaks (shaded) "coupled" or phased enabling large-scale diffluence (magenta coutours) over the northern Plains. The upper-level trough adopted a strongly-negative tilt as it progressed into the Ohio River Valley, fostering the rapid deepening of the surface low pressure system.

1:00 PM EST, Dec. 4th, 2017

300mb heights, wind speeds, and divergence

1:00 AM EST, Dec. 5th, 2017

300mb heights, wind speeds, and divergence

1:00 PM EST, Dec. 5th, 2017

300mb heights, wind speeds, and divergence

A similar evolution was evident at 500 mb, or roughly 18,000 feet into the atmosphere. Note the strong 120 knot jet streak over the central Great Lakes.

1:00 PM EST, Dec. 4th, 2017

500mb heights, temperatures, and wind speeds

1:00 AM EST, Dec. 5th, 2017

500mb heights, temperatures, and wind speeds

1:00 PM EST, Dec. 5th, 2017

500mb heights, temperatures, and wind speeds

The rapid strengthening of the storm is easily visible by examining the 500 mb height falls, which are related to changes in pressure. Notice how the height falls increased significantly with time, hinting at the rapid strengthening of the storm.

1:00 PM EST, Dec. 4th, 2017

500mb heights and 12-hour height changes

1:00 AM EST, Dec. 5th, 2017

500mb heights and 12-hour height changes

1:00 PM EST, Dec. 5th, 2017

500mb heights and 12-hour height changes

Closer to the ground (~ 5000 ft), strong warm air advection ahead of the surface low contributed to unseasonable warm temperatures on December 4th as well as rare December thunderstorms. With relatively low freezing levels, some thunderstorms produced quite a bit of lightning and even small hail. Also note the strong cold air advection, which factored into the strong winds at the surface.

1:00 PM EST, Dec. 4th, 2017

850mb heights, temperatures, wind speeds, and moisture

1:00 AM EST, Dec. 5th, 2017

850mb heights, temperatures, wind speeds, and moisture

1:00 PM EST, Dec. 5th, 2017

850mb heights, temperatures, wind speeds, and moisture

At the surface, the rapid deepening of the low pressure system was evident, dropping over 20 mb in a 24 hour period. Also notice how the surface pressure tendency couplet strengthened with time (e.g. red and blue lines). The very strong winds observed on and along the western shore of Lake Superior were enhanced by this pressure tendency couplet, which is known as the isallobaric wind. As the system moved over Lake Superior, rain quickly changed to snow from west to east. Combined with strong westerly winds, many locations along the Keweenaw Peninsula saw blizzard-like conditions.